Different active ingredients used in treatment of humans and other animals require different release profiles. For example, vaccine materials may require a pulsatile release profile subsequent to an initial delay of release, while antibiotics may require a continuous release profile subsequent to the initial delay of release. The predetermined time interval (delay time prior to release of the bioactive molecule) referred to above is dependent upon the rate of degradation of the materials and subsequent dissolution of the degradation products used for the construction of the controlled release container.
To obtain a "pulsatile" time delayed delivery system for active ingredients, it is desired that the controlled release container provide a "burst" release rather than a "continuous dribble" after the required initial time delay. Thus, containment of a bioactive molecule requires a coat or shell which remains impervious while the structure is intact but undergoing degradation.
Typical prior art methods of providing controlled release devices involve coating an active ingredient with a degradable polymer using conventional tablet coating methods. These generally involve compressing the active ingredient with an excipient and other additives into a tablet of the desired size and shape. The compressed tablet may then be coated in a pan coater by spraying with a solution or dispersion of the coating material in a solvent in an amount sufficient to give the device the required coating thickness. In an alternative coating method known as the Wurster method, the tablet is coated in a fluidised bed system.
It has been found that devices formed by these conventional techniques are most suited to short delays in release of the active ingredient. They are generally unsuitable for devices for which release of the active ingredient is desired after a longer period of, for example, about 4 to 6 weeks from administration. In addition, devices formed by conventional methods such as pan or Wurster coating, or microsphere technology, often do not provide a sufficiently well defined "burst" for administration of active ingredients such as vaccine materials.
PCT Publication No. WO 92/17165 describes a pharmaceutical or veterinary implant which is stated to release a pulse of at least one biologically active material at a controllable time interval after implantation. The implant comprises the biologically active material; an excipient comprising at least one water soluble material and at least one water insoluble material; and a polymer film coating adapted to rupture at a predetermined period of time after implantation, and wherein the excipients and polymers are biocompatible. The implants of this citation are prepared by conventional techniques from the tabletting art. Preparation methods described comprise compressing the excipients with the active ingredient into tablet-shaped solid cores using known techniques and coating with a polymer by spraying in a pan coater or a fluidised bed coater. Disadvantages with these implants is that a water insoluble non-biodegradable cage remains in the body. The implants are complicated involving complex production processes and delay times longer than about 4 weeks are difficult to achieve. The implants of this disclosure are most suited to short delays.
It is an object of the present invention to overcome or alleviate one or more of the above limitations associated with conventional controlled release devices.